Tuning systems which are capable of tuning of television signals with non-standard carrier frequencies, as well as with standard (i.e., "broadcast") carrier frequencies, include provisions for changing the frequency of the local oscillator signal in order to search a range of frequencies and provisions for detecting when proper tuning has been achieved. The latter provisions usually include apparatus for examining an automatic fine tuning (AFT) signal representing the frequency relationship between the IF signal and a nominal value, and also apparatus for determining whether or not the demodulated signal produced by the video detector contains picture synchronization ("sync") information. The test for synchronization information is performed as a verification that the RF carrier which has been found during the search is a picture carrier and not a sound carrier because the AFT signal may exhibit conditions corresponding to proper tuning in response to either a picture or a sound carrier. Such a tuning system is described in U.S. Pat. No. 4,868,892 issued to J. Tults et al. on Sep. 19, 1989.
A hardware implementation, i.e., one using dedicated logic elements, of a so-called "sync presence" detector for evaluating the output signal of a synchronization (sync) signal separator of the television receiver in order to determine whether valid picture synchronization information is present is disclosed in U.S. Pat. No. 4,364,094 issued to J. Tults on Dec. 14, 1982.
In a modern television receiver, most control functions, including tuning, are performed by a microcomputer (.mu.C). It is therefore desirable to use the .mu.C for sync presence detection also.
Most .mu.Cs include a hardware counter for performing various timer and/or counter functions. If it is possible to couple the output signal of a sync separator to the timer/counter of the .mu.C through an input port, a simple method for detecting presence of sync information utilizing the .mu.C is to enable the time/counter to count transitions of the output signal of the sync separator during a predetermined time interval. If the count reaches a predetermined number, an indication of a valid sync component is generated.
The technique described above requires a .mu.C with an "on-board" timer/counter which is externally accessible.
Even if a .mu.C includes an externally accessible timer/counter, it is usually desirable to employ the timer/counter to perform other important functions, such as time keeping and decoding of remote control IR commands, which should not be interrupted.
Another problem associated with the use of a hardware counter/timer of a .mu.C is that a seemingly correct result may be obtained under the wrong circumstances, e.g., when the output signal of the sync separator includes randomly spaced noise pulses.